The present invention relates in general to motor vehicles using shift-by-wire transmission controls, and, more specifically, to a backup system for executing a transmission shift during times that a main electrical system fails to provide power for shifting.
In a conventional automatic transmission system for a motor vehicle, a direct mechanical linkage connects a driver-operated gear shift lever through a cable to a gear shift mechanism on the transmission. In a shift-by-wire system, this direct linkage is separated and an electronic system that includes solenoids or motors produces the intended movement of the gear shift mechanism.
Shift-by-wire systems provide numerous advantages such as occupying reduced space in the passenger cabin and enabling advanced functionality such as Auto Park. With shift-by-wire technology, however, a potential loss of main (e.g., battery) power creates unique failure management situations related to the park system of the vehicle. For example, loss of battery power can result in the inability to shift the transmission as desired whenever the vehicle may be disabled and without electrical power, e.g., shifting into park to prevent vehicle movement or into neutral for vehicle towing.
As shown in commonly assigned U.S. Pat. No. 8,560,193 to Krishnaswami et al, a capacitor bank may be provided within a transmission range control module (TRCM) which is continuously charged during normal electrical system operation to maintain a reserve power to allow a shifting actuation immediately after the onset of a power failure. Thus, as the power failure occurs, the shift-by-wire system may automatically execute some desired shift event according to the circumstances of the failure. Whether or not the reserve power is used for shifting during the vehicle shutdown due to failure, the capacitor charge decays fairly rapidly (so that operation of the shift-by-wire system becomes unavailable). In order to allow subsequent shifting of the transmission range to facilitate towing and/or parking of a disabled vehicle as needed, mechanical means have been provided to allow a service operator to open the actuator and/or cable system to physically move the transmission into a desired shift range. The known mechanical means have been inconvenient, require tools, and/or cannot be easily performed by all vehicle operators.